1. Field of the Invention
The invention relates to the heat exchanger of an auxiliary heater for mobile applications, which is formed in a cup shape and has a hollow cylindrical wall section with a first end area facing the burner of the auxiliary heater, a bottom section facing away from the burner, a heat transfer medium inlet, and a heat transfer medium outlet located on the first end area, by which in operation of the auxiliary heater a fluid heat transfer medium flows through the bottom section and the hollow cylindrical wall section. Furthermore, the invention relates to an auxiliary heater with one such cup-shaped heat exchanger.
2. Description of Related Art
Auxiliary heaters for mobile applications in which a fluid heat transfer medium is used to transfer the generated heat energy are installed, for example, as auxiliary water heaters in vehicles, such as passenger cars, trucks, busses, railway cars or ships. They are generally used to heat the passenger compartment or to preheat the cooling water of the internal combustion engine of the vehicle.
A characteristic feature of an auxiliary heater with a fluid heat transfer medium is that the heat exchanger on the burner of the auxiliary heater does not heat a gas, such as the air of the passenger compartment, but a fluid, especially a liquid. This liquid can be, for example, the liquid coolant of an internal combustion engine which is preheated by the auxiliary heater. However, the fluid can also be used, with the aid of another heat exchanger, for subsequent heating of the air of the passenger compartment. Therefore, the auxiliary heaters relevant here are often used as combination devices with which both the liquid coolant of an internal combustion engine and the air in the passenger compartment are heated.
These auxiliary heaters are known (see, for example, published German Patent Applications DE 197 49 809 A1, and DE 199 34 488 A1, and U.S. Pat. Nos. 4,543,943; 4,590,888; 4,640,262; and 6,089,465). They each have a burner which is generally provided with a fire tube. In the fire tube, a flame bums in operation of the burner; its exhaust gas is used to heat the indicated fluid. To do so, the fire tube is surrounded by a cup-shaped heat exchanger which is double-walled and by which fluid is conveyed as the heat transfer medium using a circulation pump. For this purpose, in the area of the bottom section of the cup-shaped heat exchanger, a heat transfer medium inlet is provided, and in the end area of the hollow cylindrical wall section facing the burner, a heat transfer medium outlet is formed.
Therefore, the heat transfer medium flows through the known heat exchangers proceeding from the bottom section of the heat exchanger in the direction toward the end area facing the burner. This type of flow ensures comparatively good and uniform heat transfer from the exhaust gas to the heat transfer medium. So that the heat transfer medium can enter in a streamlined flow at the bottom section, known heat exchangers have a sloping bottom section. However, due to the slope of the bottom section, the heat-transferring surface on the bottom section on the flue gas side is reduced by roughly the inside diameter of the heat transfer medium inlet. This leads to reduced heat transfer, and thus, to a higher exhaust gas temperature and poorer efficiency of the heat exchanger. In addition, when the heat transfer medium enters the bottom section radially or tangentially, it is deflected into the axial extension or route of the hollow-cylindrical wall section, so that backflow and dead water areas form which promote bubbles, especially steam bubbles, and cavitation.
German Patent 1 800 561 discloses a hollow cylindrical heat exchanger with hollow cylindrical heat exchanger elements located concentrically in one another. The basic version of heat exchanger does not have a heat-transferring bottom section so that its efficiency is comparatively small compared to its size. In order to implement a heat-transferring bottom section, a single heat exchanger element must be provided there, with a separate heat transfer medium inlet and outlet. The provided heat exchanger elements located concentrically in one another, likewise, require a respective heat transfer medium inlet and outlet. In addition, the flow of heat transfer medium is deflected in narrow arcs; this likewise leads to the aforementioned problems with backflow and dead water areas.